Add project version to Markdown output

[montogeek]

Hi!

At this moment the option --project-version is only working when the output is HTML, it should be added when the output is Markdown too.

I did not test other output options.

[tmcw]

I'd accept a PR that implements optional output of the project version to Markdown output. I'm not sure that this is a good default, because my assumed usage of Markdown output is in GitHub readmes, where the documentation will be right next to the code and there will be a more reliable version indicator - a badge, git tag, package.json - nearby.